Vacuum flask



FIG. 1 is a left side view of a vacuum flask showing my new design;

FIG. 2 is a right side view thereof;

FIG. 3 is a front view thereof;

FIG. 4 is a back view thereof;

FIG. 5 is an enlarged top plan view thereof; and,

FIG. 6 is a bottom plan view thereof. 

The ornamental design for a vacuum flask, as shown and described. 